Association between Successful Palpation of the Cricothyroid Membrane and the 3-3-2 Rule for Predicting Difficult Airway in Female Patients Undergoing Non-Neck Surgery: A Prospective Observational Cohort Study

Background: Prediction of difficult airway is important for airway management in patients undergoing surgery. The assessment of airway structures and establishment of protective airway strategies are essential to improve patient safety. However, the association between successful palpation of the cricothyroid membrane and airway predictions has not been fully elucidated in patients undergoing surgery. We investigated this in female patients undergoing non-neck surgery. Methods: A total of 68 female patients were enrolled in this prospective observational cohort study between January 2021 and June 2021 at Eunpyeong St. Mary’s Hospital, College of Medicine, Catholic University of Korea, Seoul, Republic of Korea. Exclusion criteria were male patients and those with neck pathology or neck surgery. The assessment of difficult airway was performed before the induction of anesthesia and was defined by one of the following conditions: inter-incisor distance < 3 fingerbreadths, hyoid-to-mental distance < 3 fingerbreadths, and thyroid-to-hyoid distance < 2 fingerbreadths (the “3-3-2 rule”). The accuracy of palpable identification of the cricothyroid membrane was confirmed by ultrasonography (US). The patients were divided into the non-difficult airway (NDA) group (n = 30) and the difficult airway (DA) group (n = 30). Results: The two groups were comparable in terms of age, but the DA group had higher body mass index (BMI). In airway assessment, 9 patients showed inter-incisor distance < 3 fingerbreadths, 3 patients showed hyoid-to-mental distance < 3 fingerbreadths, and 24 patients showed thyroid-to-hyoid distance < 2 fingerbreadths in the DA group. The rate of successful palpation of the cricothyroid membrane was higher in the patients without than in those with difficult airway variables. Conclusions: Patients with a positive 3-3-2 rule showed a poor palpability of cricothyroid membrane.